In recent years, following the spread of portable appliances such as video cameras and laptop personal computers, there has been an increased demand for small-sized and high-capacity secondary batteries. Almost all of secondary batteries are a nickel-cadmium battery or a nickel-hydrogen battery each using an alkaline electrolytic solution. However, in such a battery, the voltage is low as about 1.2 V, and it is difficult to enhance an energy density. For that reason, studies have been made regarding a lithium secondary battery using a lithium metal, which has a specific gravity of 0.534, a value of which is the lowest among solid simple substances, is also extremely poor in a potential and has the largest current capacity per unit weight among metal negative electrode materials.
However, in secondary batteries using a lithium metal for a negative electrode, dendritic lithium (dendrite) is deposited on the surface of the negative electrode at the time of charge and grows with the progress of a charge/discharge cycle. Not only the growth of the dendrite deteriorates a cycle characteristic of the secondary battery, but in the worst case, the grown dendrite breaks through a separator that is a diaphragm disposed so as to prevent a positive electrode and a negative electrode from being in contact with each other, whereby the positive electrode and the negative electrode cause an electrical short circuit.
Then, for example, as disclosed in Patent Document 1 (JP-A-62-90863), there was proposed a secondary battery in which a carbonaceous material such as cokes is used for a negative electrode, and charge/discharge is repeated by doping and dedoping an alkali metal ion. According to this, it has been noted that the foregoing deterioration problem of the negative electrode in repeating charge/discharge can be suppressed.
On the other hand, as to positive electrode active materials, as a result of search and development of an active material exhibiting a high potential, materials exhibiting a battery voltage of about 4 V have appeared and attracted attention. As such an active material, there are known inorganic compounds such as alkali metal-containing transition metal oxides or transition metal chalcogens. Above all, LixCoO2 (0<x≦1.0), LixNiO2 (0<x≦1.0) and the like are the most promising from the standpoints of high potential, stability and long life. Of these, in particular, positive electrode active materials mainly composed of LiNiO2 are a positive electrode active material exhibiting a relatively high potential and are high in a discharge current capacity, and they are expected to enhance an energy density.
On the other hand, in a secondary battery using a positive electrode active material composed of a lithium transition metal complex oxide mainly made of nickel, there are encountered such problems that an internal pressure rises following the generation of a gas in the inside of the battery; and that in a laminate-enclosed battery, blister is easily caused. Thus, it is demanded to solve these problems.
As techniques for solving these problems, Patent Documents 2 (JP-A-6-111820) and 3 (JP-A-6-215800) propose the removal of impurities of a positive electrode active material by washing the active material with water. Also, in Patent Document 4 (JP-T-2004-533104), there is made an attempt to prevent the generation of a gas by coating LiCoO2 on a positive electrode active material made of a lithium transition metal complex oxide mainly composed of nickel.